An acceleration sensor which is one of MEMS inertial sensors generally utilized widely is roughly configured by a weight (a movable part) and a supporting beam (an elastically deformed part) and acceleration that acts on the weight is converted to an electric signal. Such MEMS inertial sensors include a capacitance type acceleration sensor that detects the displacement of the weight which is operated when acceleration acts on the weight as the variation of the capacity of a capacitor electrode (a detection electrode) configured by the weight and a fixed part, converts the variation of the capacity to an electric signal in an LSI circuit and outputs the electric signal as the output of the MEMS inertial sensor.
Opportunities that circumferential environment is recorded and the kinetic attitude of a person and a robot, a running condition of a vehicle and others are detected by combining an inertial sensor represented by an acceleration sensor with plural types of sensors increase according to the recent popularization of sensors and the divergence of product types. Besides, opportunities that sensors are utilized on inadequate conditions related to temperature, vibrational noise, electromagnetic noise and others which cannot be respectively supposed heretofore also increase according to the divergence of scenes that sensors are utilized.
For example, in an attitude control system that supports safe running by inhibiting a side skid and spin of a vehicle, MEMS inertial sensors such as an angular velocity sensor that detects angular velocity and an acceleration sensor that detects acceleration in a longitudinal direction and in a lateral direction are used as a key device. In the attitude control system, the side skid and the spin of the vehicle are inhibited by controlling the output of an engine and a braking system based upon outputs from the angular velocity sensor and the acceleration sensor.
Failure of the inertial sensor which is the key device of the attitude control system may cause a deadly accident in the running of an automobile with high probability and the possibility of the failure of the sensor is required to be inhibited as low as possible. At the same time, if failure should occur, the failure of the sensor is promptly detected and is required to be informed a high-order system such as a control system. Besides, in a mobile robot used in a field such as care and medical care, an MEMS inertial sensor is utilized as an internal sensor for recognizing a self-condition for attitude control and touch judgment. To safely and precisely touch a person, the possibility of failure of the inertial sensor which is a key device is required to be inhibited as low as possible. At the same time, if failure should occur, the failure of the sensor is promptly detected and is required to be informed a high-order system such as a control system.
Therefore, as the possibility of the failure of the inertial sensor that records circumferential environment and detects the kinetic attitude of a person and a robot, a running condition of a vehicle and others is required to be inhibited as low as possible, it is desirable that a failure diagnostic function that promptly detects the failure of the sensor if the failure should occur is provided and a result of failure determination is reliable.
For an example of the MEMS inertial sensor provided with the failure diagnostic function, a patent literature 1 can be given. In an MEMS inertial sensor described in the patent literature 1, capacitance between a movable electrode and a fixed electrode is varied by superimposing a low-frequency diagnostic signal on an ac signal applied to detect capacitance of a detection electrode. It is described that it can be also judged whether an acceleration sensor fails or not, detecting acceleration which is a detected signal by observing whether the output generated as a result of the acceleration sensor shows oscillation or not. It is described that at this time, as the low-frequency diagnostic signal oscillates the output at an amplitude to an extent that the measurement of acceleration is not hindered, constant diagnosis that it is diagnosed whether failure occurs or not, detecting acceleration is enabled. Besides, it is described that when the diagnostic signal (the diagnosis type switching signal) is at a low level, high-acceleration diagnosis that the movable electrode is greatly oscillated forcedly is also enabled.